Organic light-emitting diode (OLED) has showed great potential in applications of optoelectronic devices (such as flat-panel displays and lighting) because of the synthetic diversity, relatively low manufacturing costs, and excellent optical and electrical properties of organic semiconductive materials.
In order to improve the emitting efficiency of organic light-emitting diode, various light emitter materials based on fluorescent and phosphorescent materials have been developed. Organic light-emitting diodes using fluorescent materials are characterized by high reliability, but their internal electroluminescence quantum efficiency is limited to 25% under electric field excitation, since the probability ratio of the exciton to singlet excited state and triplet excited state is 1:3. In 1999, Professor Thomson of the University of Southern California and Professor Forrest of Princeton University incorporated tris (2-phenylpyridine) iridium Ir (ppy)3 into N, N-dicarbazole biphenyl (CBP), to successfully prepare green electroluminescent devices, which aroused great interest in complex phosphorescent materials. The introduction of heavy metals improves the molecular spin orbit coupling, shortens the phosphorescence life and enhances the intersystem crossing of molecules, so that phosphorescence can be successfully launched. Further reactions of this kind of complex are mild, thus it is easy to change the complex structure and the substituent group, to adjust the emission wavelength, and thus get the excellent performance of the electroluminescent material. So far, the internal quantum efficiency of phosphorescent OLED is close to 100%. However, the stability of phosphorescent OLED needs to be improved. The stability of the phosphorescent OLED depends largely on the luminous body itself. Most of the widely used iridium and platinum metal complexes are mostly confined to the five-membered ring ligands, while the six-membered ring ligands with structural stability are less. In order to further improve the material properties and broaden the phosphorescent metal complexes, it is urgently needed to develop highly efficient phosphorescent metal complexes containing new ligands.